Power tools, such as circular saws, typically have a handle molded into the body of the tool. Such a handle is grasped by the power tool operator to guide and propel the tool through the workpiece. Usually, in a circular saw, there is a rear handle and a forward handle. The rear handle oftentimes resembles a pistol grip, and extends upwardly and forwardly. The handle is separated from the body of the saw so that the operator can easily grasp an elongated handle section that fits comfortably within the hand of the operator. This handle section typically extends in a direction that is generally parallel to and along the line of travel of the saw. As is apparent, it is extremely desirable to have the on/off switch for the saw located where it can be actuated by at least the index finger of the operator's hand engaging the handle. Such an arrangement allows an operator to selectively start and stop the cutting operation of the saw while having his/her hand gripping the handle.
Many prior power tool constructions have a lockout mechanism also associated with the handle structure which holds the switch on the handle in a locked position and requires the operator to actuate the mechanism prior to turning the power tool to the "on" position utilizing the switch. In particular, many of these prior structures require an operator to actuate a separate button or lever with his/her thumb prior to or simultaneously with actuation of the switch by the index finger of the operator's hand gripping the handle.
Prior lockout mechanisms or latches typically are of two main types, a pivoting type and a sliding type. In a pivot-type arrangement the latch is pivotally mounted within the handle structure about an axis which is transverse or perpendicular to the elongated direction of the handle. In the case of a circular saw, the latch is pivotally mounted about an axis that is parallel to the axis of rotation of the saw blade. These latches operate by pivoting between an engaged position wherein the handle switch contacts the latch member and is prevented from movement to its "on" position, and a disengaged position wherein the operator is allowed to actuate the switch to the "on" position. Examples of these transverse pivotal lockout mechanisms can be found in U.S. Pat. No. 3,873,796 and U.S. Pat. No. 5,577,600. In each of these references, the latch mechanism is actuated by a button located on the top surface of the handle. In particular, they require either the pushing of the button or the rotating of the button rearwardly to allow actuation of the switch. These structures are disadvantageous for various reasons. In particular, the location of the lockout mechanism button on the top surface of the handle requires the positioning of the thumb in an awkward position. More specifically, it is natural when gripping a handle for the thumb to be along the side of the handle with the cross section of the handle received between the thumb and index finger. As is apparent, to actuate the mechanisms in these references, the thumb must first be positioned on the top of the handle, thus resulting in a less secure grip on the handle. Such loose gripping can result in misalignment of the saw during its initial cutting actions. Still further, in these prior references, for the thumb to reach the normal gripping position on the side of the handle, the thumb must slide off the button and over the side of the handle. The friction associated with the thumb passing over the top surface of the handle and the awkward sideward movement of the thumb can result in operator discomfort during the initial cutting action of the saw.
A still further disadvantage of these references is the location of the lockout mechanism at the same general location of or behind the location of the on/off switch with respect to the longitudinal axis of the handle. More specifically, when a person typically grabs a handle, the tendency is for the thumb to be forward of the index and middle fingers. To actuate the lockout mechanism buttons of these references, the thumb must be moved rearwardly to push the actuating button, thus presenting a potential awkward position for the saw operator, and, further, possibly resulting in unnecessary reorientation of the thumb along the side of the handle to the normal gripping position.
These references suffer from a further disadvantage in that they do not provide a "trigger" feel or structure for saw operation. More specifically, in each of these references, the trigger mechanism is pivotally mounted at a location far down the handle from the normal positioning of the index finger of the operator. The pivoting arc of such structures is relatively great and results in the trigger lever or button extending a fair distance longitudinally within the handle structure. As is apparent, to have a true "trigger" type feel to an actuating switch, and to decrease the space necessary for the switch, it may be desirable to have the pivot point for the switch located at a location adjacent the index finger of the operator's hand as it grips the handle. Thus, the rotation of a switch is truly of a "trigger" nature if the pivot point is located adjacent the top of the switch and the lower end of the switch rotates inwardly toward the handle. The large trigger structures of the above references also may result in some instability and finger fatigue in operating the structure. More specifically, because the trigger structure is not confined by a guard but extends along the length of the handle, it may be difficult for an operator to align his or her fingers with the trigger for actuation thereof.
The second type of lockout mechanism includes a latch member which, when actuated, slides within the handle housing to allow actuation of the on/off switch by the operator. An example of this type of sliding latch member is disclosed in U.S. Pat. No. 5,638,945. These sliding lockout mechanisms are oftentimes relatively complicated and do not allow ergonomic positioning of the thumb during the beginning power tool operation. More specifically, the lockout structure of the above patent, again, has the actuating button positioned on the top surface of a handle housing and at a location that is above the actuating switch for the power tool. Thus, an operator, to use the power tool, is required to position his or her thumb on the top of the handle instead of along the side, and to push the lockout mechanism button forward on the upper surface while pushing upward on the switch, and thereafter to slide the thumb of the hand positioned on the handle to the side of the handle to the normal comfortable gripping position. As with the pivoting latch mechanisms discussed above, this sliding-type mechanism is highly disadvantageous because it requires the operator to utilize significant effort to reposition his or her thumb in a normal gripping operation, and also has the sliding actuating switch or button located generally at the same location as the on/off switch along a longitudinal axis of the handle which is typically not a normal position for a user's thumb of the hand gripping the handle. An additional disadvantage of sliding mechanisms is that they are oftentimes subject to contamination by dirt or grease, which affects their operations. In particular, sliding mechanisms often have grooves and sliding surfaces which can become fouled easily.
Prior art lockout mechanisms are also oftentimes subject to substantial forces as an operator attempts to actuate the power switch with the lockout mechanism in its locked position. Sometimes, such prior art mechanisms will give way and actually allow actuation of the power switch, without the operator first utilizing the button or other structure to disengage the lockout mechanism.
Therefore, a lockout mechanism is needed which will overcome the problems with the prior art lockout mechanisms discussed above.